Cracking and Local Melting in Mg-Alloy and Al-Alloy During Friction Stir Spot Welding

Abstract

Although it is generally assumed that friction stir seam welds and friction stir spot welds are free of many of the defect formation issues commonly associated with fusion welding, liquid penetration induced (LPI) cracking in the stir zone have been recently found in friction stir spot welds of AZ91. In the present study, cracking during friction stir spot welding of Mg-alloy (AZ91, AM60 and AZ31) sections is examined. Both liquation cracking and liquid penetration induced (LPI) cracking are observed in Mg-alloy friction stir welded joints. Local melting and cracking is also apparent in Al 7075-T6 friction stir spot welds produced with the precise objective of limiting dissolution of melted eutectic films in the high temperature stir zone and when spot welds cool to room temperature. Based on these test results there is no need to assume that the stir zone temperature during friction stir spot welding is less than that required for formation of melted eutectic films or for spontaneous melting of second-phase particles contained in the as-received base material.